Maybe it's there to prevent desyncing errors that occur for long strings of zeroes. Information theory isn't the only possible consideration.

Or it could be a placeholder for future expansion and while it would do nothing now, in the future it might break things if you've ignored the documentation.

If you design a protocol without at least one field reserved for future expansion you are a bad engineer. Generally I would call this the protocol version number field, but there are other options. The important thing is there is some field that currently has a defined range, but can get an a larger range in the future and you check that field and abort if it is out of range.

I also recommend a second field called protocol ID which is set to a known random value (your wife's birthday) so that if someone gets an unknown message if they see that value they can guess what the protocol is.